This invention relates to a metal film having a uniform change of resistance with temperature and to the method of manufacturing such film. In its more specific aspect, this invention relates to resistance thermometers manufactured from high purity platinum desirably formed on a ceramic substrate in a particular configuration or pattern.
Resistance thermometers, such as those made with solid wire, measure temperature by measuring the resistance of the metallic element. That is, resistance of the metal is dependent on the temperature, and therefor measuring the resistance provides a measurement of the ambient temperature.
Thick film technology is used in the field of microelectronics for the manufacture of conductors and resistors. Typically, a precious metal or metals such as platinum, gold or silver is bonded to a substrate usually with a vitreous material. Thick film, as distinguished from a thin film, is generally considered to be a metal layer ranging in thickness from approximately 0.005 millimeters to 0.030 millimeters, and usually refers to a paste or ink applied by screen printing in the manufacture of the circuit. Thin film generally is applied by vacuum processes, and the metal layer thickness is generally expressed in angstroms and may range up to approximately 10,000 A.degree. (0.001 mm).
Thick film technology for the manufacture of thermocouples is disclosed in U.S. Pat. No. 3,099,575 to Hill. According to the teachings of this patent, one or more precious metals are suspended in an organic vehicle, and the resulting paste is printed on a substrate such as fused quartz. The film is then fired, thereby bonding the metal to the substrate and producing a thick film thermocouple. U.S. Pat. No. 3,781,749 to Iles and Selman specifically discloses a thick film metal layer structure for a resistance thermometer. Here the precious metal is bonded to the substrate with a vitreous material in an electrically conducting tortile path.
An important characteristic for a commercially useful resistance thermometer is that it have a high temperature coefficient of resistance (TCR), typically measured in parts per million per degree Centigrade (ppm/.degree. C.). For example, one standard in the industry for platinum wire resistance thermometers is 3850 ppm/.degree. C. (e.g. 0.003850 ohms per ohm-degree Centigrade). For a typical thermometer having an overall resistance of 100 ohms at the ice point (0.degree. C.), the resistance will increase by an average of 0.385 ohms for each one degree increase of temperature in a range from 0.degree. C. to 100.degree. C. A high TCR is advantageous in that the higher the TCR the greater the change in resistance per degree Centigrade, which simplifies instrumentation and lowers the costs of equipment. Pure platinum has a TCR of 3928+ ppm/.degree. C. and this value is approached by laboratory grade thermometers which may have TCR's of 3927+ ppm/.degree. C. For industrial use, numerous standards have been suggested and used. The TCR value of 3850 ppm/.degree. C. has been adopted widely in Europe and is expected to be in common use in the United States as well. It has been difficult to achieve a TCR of 3850 ppm/.degree. C. with thick films, as was disclosed in U.S. Pat. No. 3,781,749. One object of this invention is to provide a TCR of 3850 ppm/.degree. C. or higher employing thick film techniques.
The TCR of a useful thermometer must exhibit stability, which is defined as the ability to maintain its specified resistance, temperature characteristics for long periods of time while being operated within its special temperature limits. This invention has another object to provide a resistance thermometer exhibiting good stability, desirably of a configuration which permits retrofit use with the various resistance thermometers, generally cylindrical units, that are currently in use.